Intrusion detection systems (IDSs) must be capable of detecting new and unknown attacks, or anomalies. We study the problem of building detection models for both pure anomaly detection and combined misuse and anomaly detection (i.e., detection of both known and unknown intrusions). We propose an algorithm to generate artificial anomalies to coerce the inductive learner into discovering an accurate boundary between known classes (normal connections and known intrusions) and anomalies. Empirical studies show that our pure anomaly detection model trained using nor al and artificial anomalies is capable of detecting ore than 77%of all unknown intrusion classes with more than 50%accuracy per intrusion class. The combined misuse and anomaly detection models are as accurate as a pure misuse detection model in detecting known intrusions and are capable of detecting at least 50%of unknown intrusion classes with accuracy measurements between 75% and 100%per class.